In U.S. Pat. No. 3,840,389, a process for preparing catalysts for reduction of nitric oxide is taught. These catalysts include as the active component, ruthenium as well as palladium, platinum, rhodium, osmium, and iridium. The patentee teaches that the active component can be supported on any refractory oxide support, including alumina, silica, magnesia, thoria, titania, zirconia, silica-alumina, silica-zirconia, magnesia-alumina, as well as carbon, pumice and other clays, including natural and synthetic clays. The patentee teaches the equivalence of all of these support materials and only uses alumina in his examples. The novelty of the patentee's invention resides in maintaining the impregnating solution at an acidic pH of from 0.7 to 2.2. The patentee teaches that his impregnating solution may be aqueous or an alcoholic solution substituted therefor. However, because the patentee teaches us that it is critical to maintain the acidity of his impregnating solution in a 0.7 to 2.2 pH range, it is necessary that some water must be present even in the alcoholic impregnating solutions taught. The patentee only uses aqueous inpregnation techniques in his examples and nowhere teaches, shows or suggests that the use of alcohol alone would provide an improved catalyst.
The patentee, because he never actually used magnesia or magnesia-alumina as a support material, also failed to recognize that catalysts using these support materials could not be prepared by aqueous impregnation wherein the active components, especially ruthenium, is in a well dispersed state. It has now been found that when aqueous ruthenium containing solutions are used to impregnate magnesia, magnesia-alumina or other Group IIA oxide supports, the following results occur, none of which were recognized by the patentee.
1. At the pH utilized by the patentee, magnesium oxide will be attacked by the impregnating solution, dissolve to some extent, and thus raise the pH.
2. The ruthenium component, because of the increase of pH will precipitate out of solution onto the magnesia as large aggregates of ruthenium complexes.
3. Reduction of the large aggregates, to activate the catalyst, yields large aggregates of ruthenium metal which have a low surface area and consequently low catalytic activity.
The patentee also teaches that corrugated ceramic material commonly known as monoliths may be used as catalyst supports. The patentee's technique, which is an impregnation technique, does not give a uniform distribution of high surface area ruthenium on supports of this type, particularly when magnesia or any other Group IIA oxide is one of the components of the monolith which is accessible to the impregnating solution.